System and method for providing electromagnetic imaging through magnetoquasistatic sensing

ABSTRACT

A system for providing electromagnetic imaging through magnetoquasistatic sensing contains an electromagnetic sensor for imaging a sample. The electromagnetic sensor contains drive/sense electronics and a pixelated sensor array having an array of inductive loops that source magnetic fields that interact with the sample, wherein the inductive loops are individually drivable by the drive/sense electronics in a coordinated manner to establish a desired temporal and spatial pattern in which electrical properties of the inductive loops are used to generate an image. Other components of the system include a precision motion controller, sensor head and associated electronics, and a computer for performing data acquisition and signal inversion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims priority to aco-pending US non-provisional application entitled SYSTEM AND METHOD FORPROVIDING ELECTROMAGNETIC IMAGING THROUGH MAGNETOQUASISTATIC SENSING,having patent application Ser. No. 12/695,962, filed Jan. 28, 2010, andto US Provisional Application entitled, “SYSTEM AND METHOD FOR PROVIDINGELECTROMAGNETIC IMAGING,” having patent application Ser. No. 61/148,043,filed Jan. 28, 2009, both of which are entirely incorporated herein byreference.

FIELD OF THE INVENTION

The present invention is generally related to imaging technologies, andmore particularly is related to imaging technologies capable ofmeasurements in the near-surface-volume of a material.

BACKGROUND OF THE INVENTION

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Methods of measuring a sample are desirable. For example, it isdesirable to be able to measure the geometry of features on a samplewith resolution on the order of micrometers or nanometers, for instancein the case of semiconductor integrated circuits and photomasks,microelectromechanical devices, and other microstructures. Additionally,surface and subsurface objects, defects, and anomalies in a sample ordevice may result in detrimental effects when the device is used. As anexample, it is desirable to be able to detect unwanted contaminationparticles present on photoreticles (and their protective pellicles) usedin the mass production of integrated circuits as well as the detectionof other defects that might arise during fabrication of these integratedcircuits. Other examples of the usefulness of measuring samples in theintegrated circuit industry include the evaluation of buried conductortraces in multiple insulating layers and the characterization/imaging ofhigh aspect ratio isolation trenches that prevent current leakagebetween adjacent integrated circuit (IC) components. Additionally, itmay be desirable to measure magnetic and electric field patterns createdin the vicinity of operating integrated circuits.

In the early 1980's Binnig and Rohrer developed the scanning tunnelingmicroscope (STM). In this same year, Binnig, Quate, and Gerber inventedthe atomic force microscope (AFM), which is built on the principles ofthe STM. In general, an AFM works by monitoring forces between a sharpprobe tip and a sample as it is precision scanned over the surface ofthe sample. In 1984, Matey and Blanc of RCA Laboratories inventedscanning capacitive microscopy (SCM) where they utilized pre-developedinstrumentation and pickup circuitry from the RCA capacitive electronicdisc (CED) VideoDisc player. SCM is similar to AFM but specificallytargets changes in capacitance between the probe tip and the surface,and for this reason, SCM is also referred to as scanning probecapacitance imaging. These inventions in the 1980's have spawned a greatdeal of research into the use of scanning probe microscopy as a meansfor high-resolution imaging of objects at both macroscopic andmicroscopic scales.

In the most general sense, an SCM works by scanning an electricallyconducting probe over the surface of a sample. FIG. 1, which is takenfrom Applied Physics, by J. R. Matey and J. Blanc, Volume 57, page 1437(1985), illustrates the basic geometry of a prior art SCM probe head 10over a sample 14. As shown by FIG. 1, the SCM probe head 10 houses asharp tip electrode 12, which is scanned over the surface of the sample14.

An image is created by monitoring local changes in capacitance betweenthe sharp tip electrode 12 and the sample 14 or a conducting surfaceunder the sample 14. This change in capacitance serves as the contrastagent in the generated image. While there have been many differentapproaches toward improving SCM, primarily with respect to probe shape,pickup circuitry, and image reconstruction, what has remained commonamongst groups attempting to develop high-resolution nanometer scaleimaging devices via AFM SCM is the use of a single sense electrode. Awell-known and well-documented pitfall of the single sense electrode isits inherent lack of ability to shape the electric field to a desiredconfiguration in order to allow selective spatial imaging. While it ispossible to raster scan a single probe electrode over a surface withhigh lateral resolution, such a configuration does not, for example,provide information for characterizing the depth and volume of thesample in desired spatial dimensions. Consequently, the single probedesign cannot optimize spatial and depth parameters and resolution, suchas depth and width. A single probe also does not allow high imagingspeed of the area of a surface to be measured.

Magnetic scanning probe microscopes are another technology, analogous toSCM and used to image magnetic properties. Other magnetic sensors areused in the industry to detect fine patterns. For example, the magneticread head on a hard disk is used to detect very fine magnetic bitpatterns on the surface of a hard disk platter. However, arrays ofmagnetically driven and/or detected sensors have not been used togenerate high resolution two-dimensional and three-dimensional images ofsubmicron scale.

Electromagnetic electrode arrays (electroquasistatic,magnetoquasistatic, and electrodynamic) for object detection and mappinghave been used in widely different fields implementing various differentelectrode geometries. Examples include the use of circular electroderings for the mapping of biological systems inside the ring (commonlyreferred to as electrical impedance tomography (EIT)), and the use ofcoplanar, interdigital electrodes for buried object detection andnon-destructive testing. Unfortunately, electroquasistatic electrodearrays and magnetoquasistatic coil arrays have not yet been utilized togenerate high resolution, two-dimensional and three-dimensional imagesof sub-micron scale devices, such as, for example, integrated circuitfeatures.

Thus, a heretofore unaddressed need exists in the industry to addressthe aforementioned deficiencies and inadequacies.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a system and method forproviding electromagnetic imaging through magnetoquasistatic sensing.Briefly described, in architecture, one embodiment of the system, amongothers, can be implemented as follows. The system contains anelectromagnetic sensor for imaging a sample. The electromagnetic sensorcontains drive/sense electronics and a pixelated sensor array having anarray of inductive loops that source magnetic fields that interact withthe sample, wherein the inductive loops are individually drivable by thedrive/sense electronics in a coordinated manner to establish a desiredtemporal and spatial pattern in which electrical properties of theinductive loops are used to generate an image. Other components of thesystem include a precision motion controller, sensor head and associatedelectronics, and a computer for performing data acquisition and signalinversion.

Other systems, methods, features, and advantages of the presentinvention will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the invention can be better understood with reference tothe following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a schematic diagram illustrating a prior art probe headhousing a sharp tip electrode.

FIG. 2 is a schematic diagram illustrating the present system.

FIG. 3 is a schematic diagram providing an example of a drive/senseamplifier.

FIG. 4A, FIG. 4B, and FIG. 4C provide a side view, bottom view, andisometric view, respectively, of a sensor array of FIG. 2.

FIG. 5 is a schematic diagram illustrating pairs of electrodes andelectroquasistatic fields that propagate from the electrode pairs.

FIG. 6A is a schematic diagram illustrating a line array of uniformlyspaced electrodes.

FIG. 6B is a schematic diagram illustrating a grid array of uniformlyspaced electrodes.

FIG. 6C is a schematic diagram illustrating a line array with guardrings.

FIG. 6D is a schematic diagram illustrating a grid array with guardrings.

FIG. 6E is a schematic diagram illustrating non-uniformly spacedelectrodes.

FIG. 6F is a schematic diagram illustrating a non-uniformly patternedarray of electrodes.

FIG. 7A is a schematic diagram illustrating a large-square checkerboardelectrode pattern excitation.

FIG. 7B is a schematic diagram illustrating a spatial step/squarefunction.

FIG. 7C is a schematic diagram illustrating a small-square checkerboardpattern excitation.

FIG. 7D is a schematic diagram illustrating varying spatial wavelengths.

FIG. 7E is a schematic diagram illustrating spatial step/square functionwith dedicated guard rings.

FIG. 7F is a schematic diagram illustrating varying spatial wavelengthsalong one spatial dimension by different excitation patterns, which canbe extended to multiple dimensions.

FIG. 8A is a schematic diagram providing an example of individuallyaddressable electrodes where each electrode can be driven with a voltagesource.

FIG. 8B is a schematic diagram providing an example of individuallyaddressable electrodes where each electrode can be driven with a currentsource.

FIG. 9 is a schematic diagram providing a side view of drive and guardelectrodes and electric field lines generated by a sensor array.

FIG. 10A is a schematic diagram demonstrating trench detection with thepresent system.

FIG. 10B is the schematic diagram of FIG. 10A, however with differentlabeling for better understanding.

FIG. 11 is a schematic diagram illustrating electric field linesprovided by the electrode pairs of FIG. 10A and FIG. 10B, being shuntedthrough a doped silicon bulk.

FIG. 12A and FIG. 12B are graphs illustrating changes in impedancebetween a fully guarded drive/sense electrode pair as they are scannedpast trenches of various depths and widths.

FIG. 13 is a schematic diagram illustrating sensor array detection ofinformation regarding a sample at multiple depths.

FIG. 14A is a schematic diagram illustrating a first example of a sensorarray and drive/sense amplifiers when the sensor array is in a passivemode.

FIG. 14B is a schematic diagram illustrating a second example of asensor array and drive/sense amplifiers when the sensor array is in apassive mode.

FIG. 15 is a schematic diagram demonstrating use of the present imagingsystem for particle removal.

FIG. 16A is a schematic diagram illustrating an example of a cleanphotomask.

FIG. 16B is a schematic diagram illustrating an example of a photomaskhaving contaminants thereon.

FIG. 17 is a schematic diagram illustrating exciting electrodes with ashort spatial wavelength.

FIG. 18 is a further schematic diagram illustrating a contaminant on apellicle.

FIG. 19 is a schematic diagram illustrating particle detection.

FIG. 20A-FIG. 20G are schematic diagrams illustrating a MEMS fabricationtechnique for a nanoscale electrode sensor array.

FIG. 21 is a schematic diagram illustrating an array of EQS sensorelectrodes used to scan laterally past a gap in an absorber layer of aphotomask.

FIG. 22 is a schematic diagram illustrating electric field linesemanating from the EQS sensor electrodes of FIG. 21.

FIG. 23A-FIG. 23D are schematic diagrams plotting sensor electrodecurrent magnitude responses for contaminants of various ∈_(r) and σ.

FIG. 24 is a schematic diagram illustrating an array-set of sensorelectrodes.

FIG. 25 is a schematic diagram illustrating simulated geometry in apassive implementation.

FIG. 26. is a schematic diagram illustrating sample electric field linesfrom a signal trace and illustrates the coupling between the trace andelectrode array when they are in close proximity.

FIG. 27 is a schematic diagram illustrating the spatial current profilealong the sensor array for various trace depths.

FIG. 28 is a schematic diagram illustrating a magnetic field created bycurrent about a loop.

FIG. 29 is a schematic diagram illustrating excitation patterns.

FIG. 30 is a schematic diagram illustrating one embodiment of a magneticsensor head.

FIG. 31 is a schematic diagram illustrating an MQS analog to top-sideguarding.

FIG. 32A and FIG. 32B are schematic diagrams illustrating alternativearrangements involving sense winding and guard windings.

DETAILED DESCRIPTION

The present invention provides a system and method for providingelectromagnetic imaging through use of an electromagnetic sensor array.The system is also referred to herein as an electromagnetic imager.

Providing high-resolution images of a certain sample is a valuable toolfor imaging surface and subsurface features, detecting surface andsubsurface objects, detecting defects, and detecting anomalies in amaterial that would otherwise go unnoticed by other means of inspection.Such items can be located and imaged with the present system and method.Among other things, the system and method is capable of locating objectswith sub-micrometer scale resolution. This sub-micron precision, coupledwith its high sensitivity to local changes in dielectric permittivity,magnetic permeability, and electric conductivity, allows for imaging ofintegrated circuits (ICs) and other devices with features in themicrometer to nanometer regime. It should be noted, however, that thepresent invention is not limited in scale to the nanometer scale.Instead, while the following provides an example of using the presentsystem and method in the nanometer scale, there is no requirement forsuch a limitation to scale and the example is merely provided forexemplary purposes.

Examples of industry applications that can benefit from the presentsystem and method include, but are not limited to, the detection andimaging of unwanted contamination particles present on photoreticles,and their protective pellicles, used in the mass production ofintegrated circuits and the detection of other defects that might ariseduring the fabrication of integrated circuits, the evaluation of buriedconductor traces in multiple insulating layers, and thecharacterization/imaging of high aspect ratio isolation trenches. Inaddition to imaging IC features and defects with an electrically drivensensor array, a passive implementation of the same sensor array isprovided and used for both time and spatial monitoring of electric fieldand magnetic field signals in the vicinity of an operating circuit. Thefollowing provides a detailed description of active and passive sensoruse, in addition to computer simulation of results. Of course, there aremany other applications for the present system and method, and theembodiments discussed herein.

FIG. 2 is a schematic diagram illustrating the system 100 in accordancewith a first exemplary embodiment of the invention. As shown by FIG. 2,the system 100 contains a stage 110 upon which a sample 120 rests. Thestage 110 is a three-dimensional stage having three orthogonal degreesof freedom (e.g., x-y-z, R-theta-phi, r-phi-z, etc.). The stage 110 iscapable of large scale motions with high resolution, although it is notlimited to such motions. Large scale scanning/raster-scanning caninvolve moving the stage and/or the sensor head.

An electromagnetic sensor array 130 is positioned and maintained abovethe sample 120 a specific distance so as to allow for the array 130 tobe used in an active mode and passive mode. The active mode and passivemode of the sensor array 130 is described in detail hereinbelow. Inaccordance with one embodiment of the invention, features that areimaged by the sensor array 130 are small compared to the sensor array130.

The sensor array 130 is maintained a very small distance from thesurface of the sample 120. As an example, the sensor array 130 may bemaintained approximately one-hundred (100) nanometers from the surfaceof the sample 120. Of course, the sensor array 130 may be maintained adifferent distance from the surface of the sample 120. Differenttechniques may be used to maintain this small distance between thesensor array 130 and the sample 120. An example of such a technique mayinclude, but is not limited to, implementing feedback position controlvia sensing capacitance from the sample 120, sensing currents relativeto voltages of electrodes within the sensor array 130, or maintaining anaverage impedance over the sensor array 130 constant in order tomaintain the small distance (height).

The electromagnetic sensor array 130 is connected to drive/senseamplifiers 150, which are, in turn, connected to a sensor head andassociated electronics 170. The amplifiers 150 are positioned as closeto the sensor array 130 as possible so as not to amplifyparasitics/strays. These electronics could include integrated circuitryand/or surface mounted circuitry. The sensor head and electronics 170may include A/D converters, D/A converters, detectors, mixers,demodulators, feed lines, or other electronics. These electronics, alongwith the drive/sense amplifiers 150, interface the sensor array 130 andwhat is sent to a computer 200. In one embodiment, the drive/senseamplifiers and other processing electronics can be integrated within thesensor head itself, using integrated circuit fabrication techniques inconcert with microelectromechanical fabrication techniques.

Collectively, the electromagnetic sensor array, drive/sense amplifiers,and sensor head and associated electronics may be referred to as thesensor.

The electromagnetic sensor array 130 can be fundamentally eitherelectric or magnetic in character, assuming that the array size issmaller than the wavelength of light at the chosen frequency ofinterest. The two sensor arrays are well known to be duals of oneanother. The electric array contains capacitive electrodes that sourceelectric fields that interact with the sample. The magnetic arraycontains inductive loops that source magnetic fields that interact withthe sample. The important difference here is that the magnetic fieldscan penetrate conductors that the electric fields would not. Then, thetwo fields interact differently with the sample, offering differentviews of its configuration, geometry, and constituent materials.Depending upon the application, one might prefer one embodiment, theother, or both.

FIG. 3 is a schematic diagram providing an example of a drive/senseamplifier 150. It should be noted that the example of FIG. 3 is providedmerely for exemplary purposes and is not intended to limit examples ofactive mode pick-ups that may be used to pickup and measure the currenton an electrode. FIG. 3 provides the example of using a transformer andamplifier configuration to pickup and measure the current on anelectrode 132. Different operating frequencies may call for differentelectronics. It should be noted that FIG. 3 is one example of animpedance sensing circuit. Since other impedance sensing methods wouldbe known to one having ordinary skill in the art, the present inventionis not intended to be limited to the example of FIG. 3.

Referring back to FIG. 2, a precision motion controller 140 may also belocated on the sensor head 170 for ultra-fine vertical and lateralmovement of the sensor array 130. It should be noted that in anembodiment having a precision motion controller, collectively, theelectromagnetic sensor array, drive/sense amplifiers, precision motioncontroller, and sensor head and associated electronics may be referredto as the sensor.

The sensor head and electronics 170 are connected to the computer 200.The computer 200 contains several modules for performing specificfunctionality as required by the present system and method. A firstmodule 210 provides for data acquisition from the sensor head andelectronics 170.

A second module 220 of the computer 200 provides signal inversion ofdata from the sensor array 130. Such signal inversion may be provided bya signal inversion algorithm. Signal inversion is described in detailhereinbelow. The use of signal inversion methods allows for thegeneration of images, feature parameters, material properties, and otherresults.

The computer 200 may also have input and output devices 230 connectedthereto. Examples of output devices may include, for example, a monitoror printer. In addition, examples of input devices may include, forexample, a keyboard, mouse, microphone, and of course, the data from thesensor array 130. Further, input and output devices 230 may includedevices that communicate both as inputs and outputs, for instance, butnot limited to, a modulator/demodulator (modem; for accessing anotherdevice, system, or network), a radio frequency (RF) or othertransceiver, a telephonic interface, a bridge, or a router.

It should be noted that an output of the computer 200 may be raw sensordata 240 that may be transmitted to a remote location forpost-processing. Alternatively, the computer 200 may contain logictherein for allowing the processing of the raw data therein.

FIG. 4A, FIG. 4B, and FIG. 4C (collectively, FIG. 4) provide a sideview, bottom view, and isometric view, respectively, of the sensor array130. The following further describes one possible dimensional layout ofthe sensor array 130. The sensor array 130 provides forthree-dimensional sample 120 characterization and imaging. The sensorarray 130 contains a number of electrodes 132 that are arranged in amanner so as to maximize sensing performed by the sensor array 130,whether such sensing is for shallow sensing, mid sensing, or deepsensing. Arrangements of electrodes are described in further detailbelow.

FIG. 4 provides a basic electrode pattern containing a “2n×m” array ofelectrodes 132 located inside an insulating bulk. In accordance with theexample illustrated by FIG. 4, “n”=5 and “m”=3, however, it should benoted that “n” and “m” can be any real positive integers.

For nano-scale imaging, the sensor array 130 is precision fabricated inlayers. In the exemplary embodiment shown by FIG. 4, as best shown byFIG. 4B, the electrode array 130 may be fabricated in five (5) layers ofSilicon dioxide (SiO₂) (a layer of periodically spaced electrodes, alayer of SiO₂ only, a layer of periodically spaced electrodes, a layerof SiO₂ only, and finally a layer of periodically spaced electrodes). Itshould be noted, however, that the sensor array 130 may instead befabricated in a different material, as long as the material is a solidand insulating. In addition, each electrode 132 is preferably under onemicron in size, and therefore is based on a nanometer scale, although aspreviously mentioned, electrode size is not required to be based on ananometer scale.

As shown by FIG. 4B, the dimension ‘w’ represents the minimum width ofthe electrodes. The dimension ‘h’ represents the fabrication layerthickness, however, ‘w’ does not necessarily equal ‘h’. The dimension‘l’ represents the height of the electrodes 132, or the fabricationlength. After fabrication, the face that is shown in FIG. 4B, namely,the bottom view, is polished so that the face is flat. Having the faceflat provides for a more accurate maintenance of the sensor array 130distance above the surface of the sample 120 and makes the sensor array130 coplanar to planar substrates. It should be noted that there aremany other useful faces, such as, for example, ones that are stepped, orcurved if the top surface of the sample is stepped or curved.

As previously mentioned, the sensor array 130 may be used in an activemode or a passive mode. In the active mode, independent voltage orcurrent sources are used to excite the individual electrodes 132 so asto allow for sample 120 characterization at different penetrationdepths. The process of exciting the individual electrodes 132 isdescribed in additional detail below. Alternatively, in the passivemode, the electrodes 132 are not electrically driven, but instead usedto sense electric or magnetic fields from the sample 120. Both theactive mode and the passive mode are described in additional detailbelow. The following first describes the active mode.

Referring to FIG. 4, each electrode to the left of the denoted ‘symmetryplane’, shown as axis x, is excited by a sinusoidal, square, or anyother special and temporal waveforms, relative to the electrodes to theright of the symmetry plane at frequency ‘f_(o).’ With this type ofexcitation, electrodes can be categorized by one of five types: driveelectrodes, sense electrodes, guard electrodes, guard and driveelectrodes, and guard and sense electrodes. The multi-element arrays aredriven by a time-varying voltage that is applied to the electrodes in adesired pattern as a function of space, and resulting currents areacquired by the first module 210. It should be noted that arbitrary timedomain signals may also be useful, for example, to measure relaxationphenomena.

In a differential drive scheme, each electrode to the left of thesymmetry plane (x-axis) is paired with an electrode to the right of thesymmetry plane. FIG. 5 better illustrates pairs of electrodes andelectroquasistatic fields that propagate from the electrode pairs.Referring to FIG. 5, starting from closest to the symmetry plane(x-axis), a first electrode 132 a is paired with a second electrode 132aa, a third electrode 132 b is paired with a fourth electrode 132 bb,and a fifth electrode 132 c is paired to a sixth electrode 132 cc. Eachpair of electrodes is capable of penetrating the sample 120 at variousdepths based on their periodic spacing. The pair closest to the symmetryplane (x-axis) has a short depth of penetration, while the further thepair is from the symmetry plane (x-axis) the deeper the penetration ofthe quasistatic fields into the sample 120. An increasing number ofpairs being further away from each other provide an increasing depth ofpenetration of a quasistatic field into the sample 120.

As is shown by the electrode pairs of FIG. 5, the electrode pairsprovide electroquasistatic fields that extend in all directions. Withthe electroquasistatic fields extending in all directions it isdesirable to concentrate the electroquasistatic fields in a direction soas to point the electroquasistatic fields toward a sample 120.Additionally, electrodes 132 within the sensor array 130 may also act asguards. Specifically, a second electrode, either concentric or adjacentto an electrode being used to provide an electromagnetic field, acts asa guard. The guard electrode is driven at the same potential and is usedto minimize stray fields in the vicinity on the sample so that one onlyfinds fringing fields off of the end of the field-providing electrodetip since fields have been shunted out of the tips of the electrodesthrough guarding. As a result, guards force the dominant electromagneticfields to be directed toward the sample 120 that is being imaged.

Referring back to FIG. 4, in the excitation scheme, electrodes to theleft of the symmetry plane (x-axis) are drives and/or guards, whileelectrodes to the right of the symmetry plane (x-axis) are sensorsand/or guards. It should be noted that, in accordance with analternative embodiment of the invention, the positions of the drives andsensors might be switched. In FIG. 4, the electrodes labeled as Y or YYare considered to be part of an active array 134, which is a portion ofthe sensor array 130. All electrodes on the perimeter of the activearray 134, labeled in FIG. 4C with an X, should serve only as guardelectrodes and can all be driven by the same source. The purpose of theguard electrodes is to guard the non-perimeter electrodes (non-perimeterelectrodes are labeled in FIG. 4C with a Y or YY). The four pairs (or ingeneral ‘n−1’ pairs) of non-perimeter electrodes can serve as both guardand drive/sense electrodes and should each be driven by separatesources, but at the same magnitude and phase as the guard electrodes.This allows for separate monitoring of impedance variations between eachsymmetry pair of drive/sense electrodes. This excitation scheme yieldsthree fully guarded drive/sense electrode pairs and one pair ofpartially guarded drive/sense electrodes. It should be noted that thisis only one specific drive scheme, each electrode is individuallyaddressable, and there are many other drive patterns.

FIG. 6 is a schematic diagram illustrating different layouts for theelectrodes in the sensor array 130. It should be noted that the presentsystem and method may be used for many different unique layouts ofelectrodes. FIGS. 6A-6F (collectively, FIG. 6) provide examples ofdifferent sensor array layouts. Specifically, the present system may beprovided with line arrays, grid arrays, guarded arrays,coaxial/concentric arrays, non-uniformly spaced electrode arrays, arraysdesigned for locating specific features, and other arrays.

Referring to images of FIG. 6: FIG. 6A provides a line array ofuniformly spaced electrodes; FIG. 6B provides a grid array of uniformlyspaced electrodes; FIG. 6C provides a line array with guard rings; FIG.6D provides a grid array with guard rings; FIG. 6E providesnon-uniformly spaced electrodes (in this case there is a spatialgradient in electrode density, but the layout could be random); and FIG.6F provides a non-uniformly patterned array.

It should be noted that electrode excitations need not be limited to anyspecific patterns. If fact, with the many electrode layouts that mayexist, there are also many creative spatial excitation patterns.Moreover, included in this set of excitation patterns are randomexcitation patterns.

Examples of sample excitations are provided by the schematic diagrams ofFIG. 7. The figures of FIG. 7 provide the following: FIG. 7A provides alarge-square checkerboard pattern excitation; FIG. 7B provides a spatialstep/square function; FIG. 7C provides a small-square checkerboardpattern excitation; FIG. 7D provides varying spatial wavelengths; FIG.7E provides spatial step/square function with dedicated guard rings; andFIG. 7F provides varying spatial wavelengths along one spatial dimensionby different excitation patterns, which can be extended to multipledimensions.

The present system measures the current/voltage relationship at everyelectrode, where the system can drive an electrode with a voltage andsense the resulting output current, or drive the sensor with a currentand sense the resulting output voltage. It should be noted that eachelectrode may be individually addressable and detectable, as well asindividually drivable. By individually addressing electrodes, theelectrodes can be excited at the same magnitude and phase as theirneighbor electrodes. This allows electrodes to serve the dual purpose ofguarding and sensing.

In addition to a single-ended drive scheme, electrodes can be drivendifferentially in pairs. Also, not all electrodes have to be driven witha source. Some electrodes can be grounded, or left floating, and serveas dedicated sensing electrodes where the short-circuit currents oropen-circuit voltages are measured.

In a passive implementation of the sensor array 130, as described below,none of the electrodes would be driven. Rather, the electrodes would beshorted, or left floating, and serve as dedicated sense electrodes. Itis also important to note that excitation in the present system is notlimited to a specific frequency, but rather a variable frequency drivecan be used. This allows for investigation into the frequency dependentproperties of the sample, which is useful for impedance spectralanalysis.

FIG. 8 is a schematic diagram providing examples of individuallyaddressable electrodes. As shown by FIG. 8A, each electrode 132 can bedriven with a voltage source, and its current is measured to determineself-impedance. Alternatively, as shown by FIG. 8B, each electrode canbe driven with a current source and its voltage is measured to determineself-impedance. If there are n electrodes in the sensor array, thenthere are n branches in which current can flow. This creates asymmetrical impedance matrix where the diagonal terms are theself-impedance of each of the electrodes and the off-diagonal termsrepresent trans-impedances between electrodes. It should be noted thatdifferent methods of impedance detection may be used in accordance withthe present invention and are intended to be incorporated by the presentdescription. FIG. 3, which was described previously, provides oneexample of an impedance detection circuit that may be used in accordancewith the present invention.

By fully guarding the drive/sense electrode pairs in the fashionmentioned above, a majority of the electric field generated by the driveelectrodes is shunted through the sample 120. FIG. 9 is a schematicdiagram providing a side view of the drive and guard electrodes andelectric field lines generated by the sensor array 130. Shown in FIG. 9,but not in FIG. 4, are top-side guard electrodes 136. This top-sideguarding is extremely helpful in blocking stray fields from having astrong influence on the impedance or admittance between drive/sensepairs.

By shunting the electric field through the sample 120, the total currentdensity has maximum influence from the sample 120. Consequently, oneobserves maximum fluctuations in impedance/admittance between thedrive/sense electrode pairs as they scan over different features of thesample 120. It should be noted, that in accordance with an alternativeembodiment of the invention, one could implement an excitation schemethat does not utilize fully guarded electrodes, however, such a schemewould not be optimal for observing maximum impedance variations at theelectrode terminals.

The schematic diagram provided by FIG. 9 provides visualization of theimportance of fully guarding the electrode. Specifically, since most ofthe electric fields are shunted through the sample 120, the senseelectrodes become sensitive to local changes in properties of the sample120. The sensitivity of the sense electrodes makes the present systemuseful for detecting material interfaces, trenches, dust particles, andburied conductors, among other things. As stated previously, the depthof penetration of electric fields into the sample 120 is directlyproportional to spacing between electrode pairs. It should be noted thatin the present sensor array 130 one can simultaneously excite the sensorelectrodes at different spatial wavelengths without any mechanicalswitching or motion. In addition, as each electrode pair penetrates thesample 120 to different depths, the electrode pairs also serve thepurpose of guarding adjacent electrode pairs.

To demonstrate an example of use of the present system, FIGS. 10A, 10B,11, 12A and 12B exemplify simulation results of detecting air-filledtrenches in doped silicon in two dimensions (width and depth). FIG. 10Ais a schematic diagram demonstrating trench 252 detection with thepresent system 100, in addition to exemplary dimensions used for asimulation. The width of the trench 252 is labeled “w” and the depth ofthe trench 252 is labeled “d”. The number of electrode pairs used inthis specific simulation was n=3, with a first electrode pair labeled as254A and 254B, a second electrode pair labeled as 256A and 256B, and athird electrode pair labeled as 258A and 258B. This yields one fullyguarded drive/sense electrode pair, namely the second electrode pair256A, 256B.

FIG. 10B is the same schematic diagram of FIG. 10A, however withdifferent labeling for better understanding of the system 100. Thesensor array 130 contains a first guard electrode 260A and a secondguard electrode 260B. In addition, the sensor array 130 contains a firstsensor electrode 258A, a second sensor electrode 256A, a third sensorelectrode 254A, a fourth sensor electrode 254B, a fifth sensor electrode256B and a sixth sensor electrode 258B. Further, the trench 252 islocated within the sample 120. For example, the sensor electrodes ofFIG. 10B are capable of predicting the dimensions (depth and width) of asub-micrometer scale trench in doped silicon based upon a measuredtransimpedance between two sensors in the sensor array when scannedlaterally over the surface of the silicon.

Sensor electrodes 258A, 256A, 254A on the left half of the sensor array130 are driven by a sinusoidally varying voltage source. This frequencyis high enough such that it is on the order of the charge relaxationbreak frequency of the semiconducting silicon bulk, namely, the sample120. Sensor electrodes 254B, 256B, 258B on the right half of the sensorarray 130 are short-circuited to ground. The particular parameter ofinterest is the mutual transimpedance between the second sensorelectrode 256A and the fifth sensor electrode 256B.

FIG. 11 is a schematic diagram showing electric field lines provided bythe electrode pairs of FIG. 10A and FIG. 10B, being shunted through adoped silicon bulk. By viewing FIG. 11, one can see how the electricfields react to the presence of the trench 252, which ultimatelyproduces impedance variations at the electrode terminals. The electricfield plot of FIG. 11 is for a high aspect ratio trench in the siliconbulk. Electric fields from the second sensor electrode 256A are shuntedinto the silicon bulk by guarding effects from the first sensorelectrode 258A, the third sensor electrode 254A, and the first guardelectrode 260A. These field lines can be seen bending and reacting tothe presence of the air-filled trench 252. The presence of theair-filled trench 252 shows up as variations in both the magnitude andphase of the mutual transimpedance between the second sensor electrode256A and the fifth sensor electrode 256B as the sensor array 130 scanspast the trench 252. Other impedances may also be monitored.

FIGS. 12A and 12B are graphs illustrating changes in impedance betweenthe fully guarded drive/sense electrode pair 256A, 256B as they arescanned past trenches of various depths and widths. As shown by FIGS.12A and 12B, the x-axis of the graph represents horizontal trenchposition in nanometers, while the y-axis of the graph representsmagnitude. Graph lines demonstrate readings associated with trenches of50 nm, 150 nm, 300 nm, and 600 nm in depth.

It should be noted that the sensor array of the present invention iscapable of simultaneously giving information about the sample atmultiple depths with a spatial step/square excitation, as shown by FIG.13. The impedance between the closest electrodes varies most stronglywith shallow-depth variations in sample properties. A mid-distance pairreacts strongly to shallow objects, but also reacts to mid-depthvariations that the close electrodes cannot. Furthermore, a widelyseparated pair is capable of detecting deeply buried objects that theshallow and mid-spaced electrodes cannot.

As previously mentioned, the sensor array 130 may also be used in apassive mode. In the passive mode electrodes can be used to listen forfields, rather than impose fields. The passive implementation of thesensor array 130 can be used for both the time and spatial monitoring ofelectrical signals along signal traces. Instead of driving the sensorarray 130, a passive implementation leaves the electrodes 132 of thesensor array 130 unexcited. The sensor array 130 can then detect asignal by monitoring the spatial current or voltage profile that isinduced along the sensor array 130 when it is in close proximity to alive signal.

The spatial profile provides information about signal strength,location, buried depth, and other features. Such an implementation isuseful for monitoring signals in real-time and finding broken or flawedsignal traces. As an example, in the passive implementation theelectrodes act as passive listeners that allow for watching of currentsto allow for the determination of performance of an active circuit.Specifically, the electrodes detect real-time electric fields from thelive signals, while the circuit is in operation, thereby allowing fordetermination of which portions of the circuit are not working properlyor are receiving delayed signals. The passive implementation is alsoconvenient in that it does not require creative excitation and guardingschemes, but, as mentioned above, it can utilize the same exactelectrodes as the active implementation.

FIG. 14A is a schematic diagram illustrating a first example of thesensor array 130 and drive/sense amplifiers 150 when the sensor array130 is in the passive mode. In addition, FIG. 14B is a schematic diagramillustrating a second example of the sensor array 130 which shows theinternal circuit elements used in the drive/sense amplifiers when thesensor array 130 is in the passive mode. As previously mentioned, whenbrought into close proximity to a live signal, the sensor array willcouple to the electrical signals on an integrated circuit or printedcircuit. In both FIG. 14A and FIG. 14B, the sensor array 130 couples tothe voltage signal line through the electric field of the voltage signalline. The currents that are induced on the sensor array can be amplified(for example, with current-to-voltage amplifiers). It is shown in FIG.14A that the sensors are capable of passively detecting signals that runalong the surface of the sample as well as signals that are buried inthe substrate of the sample.

The current, or voltage, profile along the sensor array can provideinformation about the magnitude of the signal, timing, depth/distance ofthe signal, or simply the existence/nonexistence of the live signal. Itshould be noted that the previously mentioned properties of electrodespacing, pickup circuitry, impedance measurements, and other propertiesnot only apply to the active mode implementation of the system, but alsothe passive mode implementation.

Referring back to FIGS. 12A and 12B, it is noted that the figuresclearly indicate that scanning past trenches of various sizes and aspectratios will produce different impedance/admittance variations betweenthe drive/sense electrode pairs. This process may be referred to as the“forward problem.” The “inverse problem” is then to reconstruct an imagefrom the impedance/admittance data. Because the local impedance dependson the material properties of objects near the sensors, one can imaginethe difficulty in reconstructing a unique and correct image inmulti-layer, multi-material, inhomogeneous environments. In measuringsamples, inverse problems involve the identification of defectparameters (e.g., length, width, depth, conductivity, etc.) from ameasured signal. It is then possible to train an artificial neuralnetwork (ANN) based inversion algorithm to estimate an object based onthose parameters. There is a wide variety of sources similar to the workdone regarding the use of ANN and many other inversion algorithms formulti-channel signal inversion. As an example, tomographic imaging andinverse filtering would be equally applicable.

When imaging a sample, or surface of interest, to detect contaminationparticles, these particles will experience electromagnetic forces. Inparticular, if working in an electroquasistatic imaging mode, and if theparticles have a dielectric constant larger than free space, theparticles will experience an attractive force in the strong field regiondriven by the gradient of the electric field E.

This force may be used to attract the particles to the imagingelectrodes, or to move the particles along the surface. In this fashionthe imaging electrodes can be used to remove contaminating particlesfrom a surface, thereby performing a cleaning function. For example, inthe case of photomasks used to produce integrated circuits,contaminating particles produce errors in the circuits imaged from suchphotomasks.

The electromagnetic imager can be used to find such contaminatingparticles in an imaging mode. Then these particles can be targeted forremoval via the electrostatic force from the array of electrodes. It maybe desirable to increase the electrostatic force by using a largervoltage drive on the sensor array electrodes during such a cleaningprocess.

Such a cleaning process may also be facilitated by first charging theparticles with electrons or ions to give the particles a net negative orpositive charge. Then the particles will experience a force proportionalto the net charge multiplied by the electric field. This may yield alarger force for particle removal and may help in attracting thecontaminated particles to the electrode array.

When attempting to remove contamination particles, it may be helpful tovibrate the object surface rapidly, in order to dislodge the particlesfrom the surface. Such vibration could be accomplished by shaking theclamp or chuck that holds the object to be imaged. Alternatively,transducers could be used to induce vibrations or waves in the object tobe imaged. For example, traveling vibration waves in the object to beimaged will cause local displacements in vibration at the site of acontamination particle, causing it to be dislodged. Finally, suchvibrations may be excited directly by forces from time-varying fieldsfrom the sensor array.

FIG. 15 is a schematic diagram demonstrating use of the present imagingsystem for particle removal. FIG. 15 demonstrates pellicle attraction bythe electric field of the sensor array where force on a dielectric isproportional to gradient of magnitude of E squared. The sensor array 130is used to control the pellicle position. In addition, electric fieldforces are used to clean the pellicle by attraction, which may be helpedby pre-charging particles 302 with ions of positive or negativepolarity. In addition, particle transfer can be detected by change incapacitance, conductance, and/or impedance. The following furtherdescribes FIG. 15 in detail.

When imaging a flexible substrate, such as the pellicle 300 of anintegrated circuit photomask, forces exerted by the sensor arrayelectric field will cause the pellicle 300 to deflect as a membrane.These forces can be used to actively control the pellicle deflection inorder to control the sensor array 130 working distance to the pellicle300. Such forces might also be used to vibrate the pellicle 300 at adesired vibration frequency in order to facilitate particle 302 removal.

The electromagnetic imager design supports massive parallelism. That is,the electrode array can be made as large as desired in order to increaseimage throughput. For example, designs using thousands or millions ofimaging electrodes or coils would allow parallel collection of thousandsor millions of channels of imaging data, and thereby allow rapid imagingof a surface of interest.

Speed of detection of a defect or contamination particle could beenhanced also by first conducting a rapid survey with the probe elementsdriven with a lower spatial frequency excitation pattern, which therebyprojects further from the array surface, with lower spatial resolution.Anomalies detected in this fast survey can then be imaged in more detailusing a higher spatial frequency excitation pattern. This process wouldallow imaging time to be concentrated on the features or defects ofinterest.

In the case of detecting defects in an artifact such as a photomask orintegrated circuit, the speed of detection may be augmented by workingdirectly in the space of the raw imager data. For example, if the rawimager data is compared against the data from a known-good artifact, adeviation in this data can be detected directly. This can speeddetection because the use of inverse algorithms is not required. Thatis, speed is enhanced by not undertaking the conversion from raw sensordata to image data.

The electromagnetic images can be used to scan biological materials,polymers, and plastics, and other materials with a dielectric constantand conductivity different from that of free space. It is also possibleto use the electromagnetic images in operation in air, other gases suchas helium or hydrogen, liquids such as water, or in vacuum or ultra-highvacuum. Imaging of bio-samples such as cells, viruses, cell components,and DNA is also possible. If these are imaged in an appropriate liquid,it is possible to image features of living cells.

As previously mentioned the sensor array can be used to detectcontaminant particles on the surface of the sample, such as, forexample, contaminants on a photoreticles or pellicle. FIG. 16A is aschematic diagram providing an example of a clean photomask, while FIG.16B is a schematic diagram providing an example of contaminants on aphotomask.

By exciting the electrodes with a short spatial wavelength, as shown byFIG. 17, the range of evanescent fields of the electrodes 132 arelimited. This forces the electrodes to react only to shallow/nearsurface variations. Ultimately, it is desirable to see fields reactingto particles on the surface of the mask/pellicle, but not deeplypenetrating the bulk. This involves appropriately setting the scanheight and electrode spacing. FIG. 18 is a further schematic diagramillustrating a contaminant 302 (particle) on a pellicle 300 and FIG. 19demonstrates particle detection. It should be noted that another way toremove contaminant dielectric particles could be using a linearlytraveling wave electric field.

In accordance with one embodiment of the invention, the drive/senseamplifiers can be integrated circuitry on the silicon substrate, orsurface mount electronics. FIGS. 20A-20G provide schematic diagramsillustrating a MEMS fabrication technique for a nanoscale electrodesensor array. As shown by FIG. 20A, the process begins with a Siliconsubstrate 310. An optional sacrificial nitride layer 312 is thendeposited on the Silicon substrate 310 (FIG. 20B) after which an oxidelayer 314 is deposited (FIG. 20C). A metal electrode pattern 316 isdeposited on the oxide layer 314 (FIG. 20D). As shown by FIG. 20E, oxideis deposited for insulation between electrodes, after which additionalmetal electrode patterns may be provided to result in a desired numberof electrodes. As shown by FIG. 20F, excess field oxide is etched away.An end face may then be created by lapping and polishing. Deepreactive-ion etching can be used to create an electrode array byremoving the Silicon substrate. It should be noted that electronics canbe microfabricated into the same substrate as the electrodes to maximizesignal integrity and minimize noise.

FIG. 21 is a schematic diagram illustrating an array of EQS sensorelectrodes 600A-600D used to scan laterally past a gap 610 in anabsorber layer 622 of a photomask 624. There are four EQS sensorelectrodes 600A-600D in a sensor head 630 and two dedicated guardelectrodes 640A, 640B. Within the gap 610 is a contaminant particle 642.The sensor head 630 is scanned at a fixed height over the absorber layer622. Contaminant particles of various different conductivity andpermittivity might be located in the absorber layer gap 610 or on top ofthe absorber layer 622. Detecting a contaminant stuck in an absorberlayer gap 610 represents a worst-case scenario since it is as far awayfrom the sensor head 630 as possible.

FIG. 22 is an electric field plot illustrating electric field linesemanating from the EQS sensor electrodes of FIG. 21. For exemplarypurposes, a dielectric contaminant with ∈_(r)=4 is positioned 200 nm tothe right of the senor head 630 center. The electric field lines fromthe first sensor electrode 600A are primarily vertical due to guardingeffects caused by the second sensor electrode 600B and the firstdedicated guard electrode 640A, which are driven at the same magnitudeand phase as the first sensor electrode 600A. Since the absorber layer622 is an excellent conductor, electric fields do not penetrate thesurface of the absorber layer 622, but rather terminate on the surface.The high density of field lines located between the second sensorelectrode 600B and the third sensor electrode 600C show that they arestrongly coupled to each other in this region. Consequently, one canexpect the second sensor electrode 600B and the third sensor electrode600C to be poor at detecting the contaminant particle 642 when stuck inthe absorber layer gap 610. Only a small portion of the fringing fieldsof the sensor electrodes 600B and 600C will couple to the contaminantparticle 642. This manifests itself as only a weak change in currentwhen these two sensors (600B and 600C) scan past the contaminantparticle 642. Some of the fields from the fourth sensor electrode 600Dare shown to pass through the dielectric contaminant particle 642, whileothers bend and terminate on the sidewalls of the absorber layer 622.

In accordance with one example of the invention, each sensor electrodecan be driven by a sinusoidally varying voltage source. The third andfourth sensor electrodes 600C, 600D and the second guard electrode 640Bshown in FIG. 22 are driven 180 degrees out of phase from the first andsecond sensor electrodes 600A, 600B and the first guard electrode 640Aso as to create a spatial square wave alternating in time. The magnitudeof each of the four sensor electrode currents as a function of sensorhead position are monitored. It should be noted that sensor electrodescould also be driven by some other time-varying waveform that is notsinusoidal.

For exemplary purposes, each of the sensor electrode current magnituderesponses are plotted in the graphs of FIGS. 23A-23D for contaminants ofvarious ∈_(r) and σ. Due to symmetry of the sensor head geometry, theresults for the first sensor electrode 600A and the second sensorelectrode 600B are identical, but mirror images, of the results for thefourth sensor electrode 600D and the third sensor electrode 600C,respectively. The first sensor electrode 600A and the fourth sensorelectrode 600D react more strongly to the absorber layer gap 610 thanthe second sensor electrode 600B and the third sensor electrode 600C.With no contaminant present, the first sensor electrode 600A seesapproximately seven percent change in signal magnitude from the baselinevalue of 0.083 [A/m], while the second sensor electrode 600B yields onlyabout a one and a half percent change in signal magnitude. In caseswhere the contaminant particle 642 is a strong dielectric (∈_(r)=10) orhighly conducting (σ=1e6 [S/m]), the sensor electrodes can distinguishbetween a contaminated and clean gap with a difference in signalmagnitude of four percent or more from what is expected for a clean gap.In the cases where the contaminant is a poor dielectric or is weaklyconducting, the contaminant becomes increasingly more difficult for thesensor electrodes to distinguish between a contaminated and cleanphotomask.

The sensor electrodes shown in FIG. 21 and FIG. 22 can be part of aneven larger array-set of sensor electrodes. FIG. 24 is a schematicdiagram illustrating such an array-set of sensor electrodes 650. Thearray-set 650 contains a series of sensor electrodes 652 and two guardelectrodes 654. The cross-section of the sensors and guards along axis Ais equivalent to a bottom view of FIG. 21 and FIG. 22. Axis B provides asymmetry plane where everything to the left of the plane is driven atthe same electric potential, while everything to the right of the planeis driven at the same electric potential.

Cross-section of the sensors and guards along axis B would representthose along line (A) of the figure. It should be noted that there canthen be “n” many stacks of sensor electrodes in the sensor head. Thesesensor electrodes can form an “Electroquasistatic Brush” that “combs”over the surface of a photomask or other substrate for contaminants ordefects with enormous parallelism.

FIG. 25 illustrates simulated geometry in a passive implementation, forexemplary purposes. An array of eight electrodes is used and no top-sideguard electrodes are present. While one could use an array that hastop-side guard electrodes present, it is not necessary since this is apassive implementation. The signal line is excited by an AC signal of 1Vat 100 MHz. The line is buried at five different depths: 0 μm (shown), 1μm, 2 μm, 3 μm, and 4 μm. The electrodes 132 in the array 130 aregrounded and their short circuit currents are monitored. FIG. 26 showssample electric field lines from the signal trace and illustrates thecoupling between the trace and the electrode array 130 when they are inclose proximity. FIG. 27 shows the spatial current profile along thesensor array for various trace depths. One can see that as the tracegets further from the sensor array, the profile begins to both smear outand decrease in amplitude.

As previously mentioned, the present system and method may also containa magnetically driven electromagnetic sensor array. Themagnetoquasistatic (MQS) analog to an individual electroquasistatic(EQS) sensor is a small and individually addressable conducting loop.Current about the loop creates a magnetic field much like charge on theelectrode creates an electric field, as shown by FIG. 28. As shown byFIG. 28, the loop 700 can have multiple turns, and can have ahigh-permeability core 702. In addition, the loop 700 can have multiple(primary and secondary) windings. Further, small permanent magnets canbe implemented to generate the magnetic fields (not shown).

As previously mentioned, there may be various excitation patterns forEQS sensor electrodes. The same applied to MQS sensor electrodes. Themapping from EQS to MQS is that the polarity of charge on an EQS sensormaps to the direction of current flow on an MQS sensor. A positivepolarity EQS sensor is equivalent to an in-plane clockwise current on anMQS sensor, and a negative polarity EQS sensor is equivalent to anin-plane counterclockwise current on an MQS sensor. This is shown by theexcitation patterns of FIG. 29, although it should be noted that suchpatterns are not limited to the example of FIG. 29.

It was also previously mentioned that one can achieve multiplepenetration depths into a sample by use of EQS sensor electrodes. Thesame applies to MQS sensor electrodes 710, as shown by FIG. 30, inaddition to a confirming finite element simulation. FIG. 30 shows theMQS dual of an EQS sensor array. The direction of the current in thecoils is chosen to match the polarity of the EQS sensor electrodes toachieve a similar field pattern. In addition, FIG. 30 illustrates theinherent dual-purpose of both guarding and sensing that coils serve whendriven with a current at the same magnitude and phase as neighboringcoils. Adjacent coils driven with currents of the same magnitude andphase act in the same way as guard electrodes driven with voltages ofthe same magnitude and phase.

FIG. 30 shows one embodiment of a magnetic sensor head above a sample.Each pair of “dot” and “cross” circles represent side views ofindividually addressable current loops that are MQS dual of an EQSelectrode. The polarity of the “dot” and “cross” determine the directionof the electric field.

The windings in the embodiment are linear, and extend into the z-axis,forming a sensor head that is used to scan sideways. The sensor heads750 could also be stacked in the z-axis to add a two-dimensionalcharacter to the sensor head 750, allowing one mechanical pass of thehead to separately sense different parallel strips of the sample. Thiswould increase the sensing rate.

FIG. 30 shows three pairs of windings labeled 710A and 710AA, 710B and710BB, and 710C and 710CC; the members of each pair could be connectedeither in parallel or series so as to adjust their impedance to bestmatch that of the drive electronics. Winding pair 710B and 710BB are theprimary sense windings. The magnetic field lines linked by this pairpenetrate deeply into the sample, and do not link fringing fields at thesides or center of the sensor head 750. Winding pairs 710A and 710AA,and 710C and 710CC, are guard windings, and link the fringing fields.The fringing fields add to the inductance of the winding pair yet do notgive a focused view of the sample, so it is desirable for the sensewindings not to link these field lines.

Herein, it has been described how guarding the top side of the EQSsensor electrodes is beneficial for shunting the fields through thesample. Shown in the schematic diagram of FIG. 31 is an MQS analog totop-side guarding (both a sketch and a finite element simulation forconfirmation). A high-permeability, high-conductivity guard material isplaced above the sensors. The highly conducting material preventsmagnetic fields from penetrating its surface. Rather, the fields areforced to bend and flow tangentially along its surface. This acts as atop-side guard as it steers the fields, preventing them from fringingout to the top-side in the same way they fringe out on the bottom side.FIG. 31 illustrates how the fields do not stray above, but rather areconfined to the area just below the top-side guard. Below the sensorarray, the magnetic field lines are free to fringe out forfeature/object detection.

FIG. 32A and FIG. 32B show an alternative arrangement involving sensewinding 800 and guard windings 802L and 802R. The current pattern inthese windings is anti-symmetric, where as the current pattern in thewindings in FIG. 30 is symmetric. As a consequence, the central guardwinding is not necessary in FIG. 32, and the flux lines linked by sensewinding 800 do not penetrate as deeply into the sample. Thus, thisarrangement of windings and current offers an alternative view of thesample.

The electromagnetic imager is configured to have a spatially distributedarray of electrodes or coils, wherein the electrodes are used forelectroquasistatic imaging, and the coils are used formagnetoquasistatic imaging.

For example in the electrostatic imager, the electrode pixelatedconfiguration could be arranged in a linear array of electrodes so as toform an electronic brush. This electronic brush is scanned across asurface to be imaged in a direction transverse to the linear array, soas to rapidly image a swath of surface, with each individual electrodeimaging element (pixel) providing high spatial resolution along thelength of the array and the scanning motion providing high resolutionimaging in the direction of the scan, based upon choice of scan velocityand imaging bandwidth. During this motion, the pixelated sensor head ismaintained at some desired distance, for example 50 nm to 100 nm, fromthe object surface via feedback control on sensor variables such as theelectrode impedance or coil inductance, taken at single or multiplepixels, or averaged over some function of pixels, or via separateconventional sensing mechanisms such as a capacitance probe, inductanceprobe, optical interferometer, near field optical sensor, or vacuumgauge.

We can regard the individual electrodes as pixels on the sensor array,and thus a pixelated electrostatic sensor array is provided. Suchpixellation is also applicable to arrays of coils for magnetoquasistaticsensor arrays, and thus the individual coil dimensions will define themagnetic pixel dimensions, and thus a pixelated magnetic sensor array isprovided. In the following discussion, we focus on theelectroquasistatic array. The extension to the magnetoquasistatic arraycan be made analogously to define an equivalent magnetic brush. Such amagnetic brush can be scanned and controlled for imaging in a manneranalogous to the control and imaging of the electrostatic brush.

For rougher surfaces, or surfaces with significant curvature, the sensorspacing from the sample could be made larger, for example, 1000 nm. Thedesired pixel size can be chosen on the basis of intended spatialresolution and sensitivity. The electrode or coil arrays are typicallydriven at a high frequency, for example 1 GHz to 30 GHz in order toachieve high sensing bandwidth. It is desirable in many cases to achievehigh sensing bandwidth to allow high imaging speed and productivity.This bandwidth can be electronically or digitally filtered to allow theselection of desired frequency content, for instance to filter out noiseartifacts. This frequency can also be adjusted to allow sensing thesample properties at some desirable frequencies on the basis of, forexample, electrical permittivity, conductivity, or magneticpermeability. The frequency of excitation can also be swept during asingle pixel imaging process to create a spectral analysis of thesurface electrical properties. This might be useful for instance todetermine the material properties of the surface.

In an examplary embodiment, a linear array could be composed of 10,000electrode sets, each with sense and guard components, wherein theindividual electrodes (pixels) could have dimensions of 100 nm by 100 nmon the face of the sensor. Thus the pixel dimensions are 100 nm by 100nm in the imaging process. As a further example, if the electrodes havepixel dimensions of 50 nm by 50 nm, and are spaced with a linear pitchof 80 nm, then an array of 10,000 electrode sets will span a length of800,000 nm or 0.8 mm. As this electronic brush is scanned in thetransverse direction, an image of 10,000 pixels wide, equivalent to 0.8mm width, is created during the scanning operation. If a data point istaken for each pixel during every 80 nm of transverse scan distance,then the image will be formed of image elements of 80 nm by 80 nm, overa width of 0.8 mm, and over the length of the transverse scan.Additional scans can then be performed in an adjacent fashion so as tocover the full surface of a sample to be imaged. For example, asuccessive scan can be performed at a spacing of 0.7 mm laterally fromthe previous scan. Measurements of lateral motion and scan motion, aswell as image correlation algorithms can then be used to stitch togetherthe successive scans in order to build up a full surface image. Forexample, with a 0.8 mm array width, 10,000 elements wide, and with 80 nmby 80 nm pixels, if the transverse scan velocity is 80 mm per second,then an imaging rate of 1,000,000 spatial samples per second can beachieved for each pixel. Over the lateral array width, this results in atotal imaging rate of 10^10 pixels per second. With such a scanvelocity, and with stitching together successive scans, a substrate of80 mm by 80 mm could be imaged in about 100 successive scans, each scanpass requiring about 1 second scan time. Allowing time for theturnaround accelerations at the end of each scan, and for scan overlap,such a substrate could then be completely imaged with 80 nm by 80 nmresolution in a total scan time of about 200 seconds. Larger and smallersubstrates would require correspondingly larger and smaller time tocomplete imaging, in proportion to the substrate area. Similarcalculations apply to a linear array of magnetic sensing coils, and thusare applicable to a magnetic brush configuration.

The electronic brush configuration is primarily a line array forscanning laterally. Another embodiment uses a grid array ofelectrostatic or magnetic electrodes or coils to form a grid of imagingpixels, a grid pixelated array. Such a grid array can be used to image asubstrate with all image elements acquired in parallel. This allows afaster imaging process of a given area of substrate. For example, anarray of 1000 by 1000 electrode or coil pixels, with 100 nm spacing, canacquire an image of an area of 0.1 mm by 0.1 mm within a time on theorder of 1 microsecond. Similarly, an array of 100,000 by 100,000 pixelswith a 100 nm spacing can acquire an image of an area of 10 mm by 10 mmwithin about 1 microsecond.

Thus, multiple substrates can be scanned very rapidly. This would allow,for instance, the creation of very high-speed high-resolution imagesequences (movies) of surfaces and samples of interest. For instance, itwould be possible to visualize high-speed processes on a surface orsubstrate, for example, biological or chemical processes, forming orerosion of surface features, or other physical changes of interest. Suchimaging would include internal features via field shaping of the array.A grid pixelated array could also be scanned over a surface to build uplarger images, in a fashion analogous to the electronic or magneticbrush. Further, the pixel shapes can be adjusted to allow imaging ofdesired surface shapes with high image fidelity and resolution.

Arrays can similarly be fabricated on spherical, cylindrical or othernon-planar surfaces in order to image samples with shapes other thanplanar, or to achieve other imaging characteristics. For example, thesurface of a convex lens element could be scanned by an array fabricatedon a concave surface. As another example, the linear array of anelectronic brush could be fabricated on an arc in order to image acylindrical surface, or to allow variable spacing from a planar surface.

It should be emphasized that the above-described embodiments of thepresent invention are merely possible examples of implementations,merely set forth for a clear understanding of the principles of theinvention. Many variations and modifications may be made to theabove-described embodiments of the invention without departingsubstantially from the spirit and principles of the invention. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure and the present invention.

We claim:
 1. An electromagnetic sensor for imaging a sample, comprising:drive/sense electronics; and a pixelated sensor array having an array ofinductive loops that source and/or sense magnetic fields that interactwith the sample, and wherein the inductive loops are individuallydrivable by the drive/sense electronics in a coordinated manner toestablish a desired temporal and spatial pattern in which electricalproperties of interaction between the inductive loops and the sample areused to generate an image.
 2. The sensor of claim 1, wherein theinductive loops are microfabricated on the same substrate as thedrive/sense electronics.
 3. The sensor of claim 1, where in a passivemode, at least one of the elements within the array of inductive loopsindividually senses magnetic fields from the sample by monitoring aspatial current or voltage profile induced along the array of inductiveloops when the array of inductive loops is in close proximity to asignal provided by the sample.
 4. The sensor of claim 1, wherein thearray of inductive loops is arranged as a linear array having sense andguard components, forming a magnetic brush.
 5. The sensor of claim 1,wherein the array of inductive loops is arranged as a grid array havingsense and guard components, forming a magnetic brush.
 6. The sensor ofclaim 1, wherein at least one inductive loop within the array ofinductive loops does not have the same dimension as other inductiveloops within the array of inductive loops, resulting in pixels havingdifferent shapes and/or sizes.
 7. The sensor of claim 1, wherein thearray of inductive loops is used to detect a variation of properties ofthe sample and to predict dimensions and/or parameter values of thevariation based upon a measured transimpedance between sensors withinthe array of inductive loops.
 8. The sensor of claim 1, wherein thearray of inductive loops is arranged to provide a sensor array layoutselected from the group consisting of a line array, a grid array, aguarded array, a coaxial/concentric array, a non-uniformly spaced array,and an array designed for locating specific features of the sample. 9.The sensor of claim 1, wherein at least one of the inductive loopswithin the array of inductive loops is individually addressable anddetectable.
 10. The sensor of claim 1, wherein the inductive loops aremicrofabricated.
 11. The sensor of claim 1, wherein windings of theinductive loops are linear and extend into a z-axis, forming a sensorhead that is used to scan sideways.
 12. The sensor of claim 11, whereinmore than one sensor head is stacked in the z-axis to add atwo-dimensional character to the sensor head, allowing one mechanicalpass of the sensor head to separately sense different parallel strips ofthe sample.
 13. The sensor of claim 1, wherein a high-permeability,high-conductivity guard material is located above the pixelated sensorarray, preventing magnetic fields from penetrating the guard material,but instead bend and flow tangentially along the surface of the guardmaterial.
 14. The sensor of claim 1, further comprising a computer incommunication with the drive/sense electronics and the pixelated sensorarray.
 15. The sensor of claim 14, wherein the pixelated sensor array ismaintained at a desired distance from the sample via the computerproviding feedback control on sensor variables.
 16. The sensor of claim15, wherein the variables are selected from the group consisting ofelectrode impedance and coil inductance, taken at single or multiplepixels, or averaged over some function of pixels, or via separateconventional sensing mechanisms.
 17. The sensor of claim 1, wherein thepixelated sensor array is maintained a distance from the surface of thesample by implementing feedback position control via sensing capacitancefrom the sample.
 18. The sensor of claim 1, wherein the pixelated sensorarray is maintained a distance from the surface of the sample bymaintaining an average impedance over the sensor array constant in orderto maintain the distance.
 19. The sensor of claim 1, further comprisinga computer connected to the drive/sense electronics, wherein thecomputer contains a first module for data acquisition and a secondmodule for signal inversion of data from the pixelated sensor array. 20.The sensor of claim 1, wherein the pixelated sensor array is used in themass production of integrated circuits to detect defects that mightarise during fabrication of the integrated circuits.
 21. The sensor ofclaim 1, wherein the pixelated sensor array size is smaller than thewavelength of light at a chosen frequency of interest.
 22. The sensor ofclaim 1, wherein a drive pattern is used to control depth of penetrationinto the sample.